1. Field of the Invention
This invention relates to digital signal recovery apparatus and, more particularly, to such apparatus in which the error rate of an equalized digital signal is reduced by minimizing high frequency noise in the digital signal. The invention finds particular application to digital recording such as a digital video tape recorder.
2. Description of the Prior Art
In a typical digital transmission system wherein digital information from a source eventually is received at another location or at another time, digital data usually is represented by a particular code. Examples of codes that have been used in various types of digital transmission systems include (1,7) codes, MFM codes, Manchester codes, and the like. Many of these codes have been developed for and continue to be quite useful in storage systems, such as magnetic digital storage devices. Such codes have been developed to prevent the original, useful digital data from being obliterated or rendered non-recoverable by the inherent characteristics of the transmission (of storage or recording) system.
It is expected that digital data will be subject to distortion as it is transmitted over a typical digital transmission system. For example, in the environment of digital recording, it is not unusual for digital data to be recorded serially by bit; and to experience distortion and interference, such as interbit interference, due to the inherent magnetic characteristics of the recording system, particularly when the digital data is played back. To account for such distortion, the encoded digital signals often are pre-coded before being transmitted (or recorded), such that predicted distortion or interference will complement the pre-coding. Hence, the combination of the pre-coding and inherent distortion characteristics will result in a digital signal from which the original digital information can be easily recovered.
The receiver (or data reproducer) of a digital transmission system often is provided with a linear equalizer which carries out high frequency compensation on the received (or reproduced) digital signal. Of course, this compensation is intended to match, or balance, the characteristics of the transmission system. The compensated digital signal then is decoded to recover the original digital information. One type of decoder that can be used when superimposed noise is expected is a so-called Viterbi decoder which functions to sample the compensated signal, correlate the resultant samples and produce digital information as a function of such correlation. A Viterbi decoder is described in "Viterbi Algorithm," by G. D. Forney, Jr., Proc. of IEEE, Vol. 61, No. 3, March 1973, p. 268 et seq. In the environment wherein so-called white Gaussian noise is expected, this type of correlation decoder operates satisfactorily. This is because white noise essentially is not correlatable with useful information.
However, if substantial high frequency noise is present on the received (or reproduced) digital signal, high frequency compensation tends to emphasize not only the useful digital signals but also the superimposed high frequency noise. Since high frequency noise correlates with useful digital information, correlation decoding is subject to errors, thereby degrading the error rate of the overall digital transmission system.